1. Field of the Invention
This invention relates to integrated solid oxide fuel cell systems and thermal management thereof using gas flow panels that absorb thermal radiation from one or more fuel cell stacks. The systems contain at least one gas flow panel that provides oxidant preheating. The systems may also contain additional gas flow panels that provide further oxidant preheating, fuel preheating, fuel pre-reforming, fuel reforming or fuel desulfurization.
2. Description of Related Art
Generally, fuel cell electrical output units are comprised of a stacked multiplicity of individual fuel cell units separated by bi-polar electronically conductive separator plates. Individual fuel cell units are sandwiched together and secured into a single-staged unit to achieve a desired fuel cell power output. Each fuel cell unit generally includes an anode electrode, a cathode electrode, a common electrolyte disposed between the anode electrode and the cathode electrode, and fuel and oxidant gas sources; and the bi-polar separator plate is disposed between the anode electrode of one fuel cell unit and the cathode electrode of an adjacent fuel cell unit, forming an anode reactant chamber on the anode electrode side of the separator plate between the separator plate and the electrolyte and a cathode reactant chamber on the cathode side of the separator plate between the separator plate and the electrolyte of the adjacent fuel cell unit. Both fuel and oxidant gases are introduced through manifolds, either internal or external to the fuel cell stack, to the respective reactant chambers between the separator plate and the electrolyte.
Solid oxide fuel cells have grown in recognition as a viable high temperature fuel cell technology. For one thing, there is no liquid electrolyte with its attending metal corrosion and electrolyte management problems. Rather, the electrolyte of the cells is made primarily from solid ceramic materials so as to be able to survive the high temperature environment, typically as high as about 1000° C. The operating temperature of greater than about 600° C. allows internal reforming to convert hydrocarbon fuels into the hydrogen fuel required for the reaction, promotes cell reactions with non-precious materials, and produces high quality by-product heat for cogeneration or for use in a bottoming cycle. The high temperature of the solid oxide fuel cell, however, places stringent requirements on its materials. Because of the high operating temperatures of conventional solid oxide fuel cells, the materials used in the cell components are limited by chemical stability in oxidizing and reducing environments, by chemical stability of contacting materials, by conductivity, and by thermo-mechanical compatibility.
Planar solid oxide fuel cells have the potential to be more efficient and lower in cost than tubular designs because the cells used have shorter current paths and are simpler to manufacture. However, as suggested above, it is difficult to find suitable low-cost materials for the sealant and interconnect for use at the solid oxide fuel cell operating temperature. Thus, to enable the use of lower cost materials, it is desirable that the operating temperature of the solid oxide fuel cells be reduced.
Effective heat integration between fuel cell stack heat removal and oxidant (air) preheating has been a major challenge for the solid oxide fuel cell. Standard heat-integration schemes, employed by conventional systems, use the cathode gas inside the fuel cell stack for the heat removal and preheat the air feed by gas-to-gas heat exchange with the cathode exhaust gas. Because the temperature gradient across the stack hardware is limited (usually less than about 100° C.), the required cathode flow for the stack heat removal is very large. Typically, a stoichiometric air ratio of 4-10 (depending upon the fuel, fuel processing, and stack size, design and performance) is necessary to provide the cathode gas flow required for the heat removal. This large air flow significantly increases the air preheater size. The large size, in conjunction with the high air discharge temperature required, significantly increases the air preheater cost. This is one major reason for the high cost of solid oxide fuel cell systems. In addition, the large air flow increases the system pressure drop. The combined effect of large flow and high pressure drop increases the air blower size and the auxiliary power consumption. Consequently, the efficiency of the system is reduced.
In larger solid oxide fuel cell (SOFC) systems, the “direct” (e.g. by radiation) transfer of stack-generated heat minimizes the airflow required to cool the stack because heat is transferred from the outside walls of the stack and, thus, reduces the size of components such as piping, ducts, and heat exchangers that use this airflow. Increased power density may also be possible because radiant heat transfer may be able to accommodate higher heat production at higher power density, which reduces capital cost. The parasitic power to run air blowers is also reduced, increasing system efficiency. In addition, the reduced pressure drop in the air passages and across the air compartments due to lower reactant gas flow in larger systems may improve seal durability and allow more flexibility in sizing the gas flow channels.
In smaller solid oxide fuel cell systems (estimated to be <3 kW), heat lost to the surroundings can become a significant fraction of the heat released by the stack. This heat loss increases the difficulty of sustaining the stack temperature, may add to burner duty and lowers system efficiency.
Planar solid oxide fuel cell stacks tend to develop an in-plane, spatial temperature distribution during operation. The temperature gradient is higher for some flow patterns than others. It is particularly high for the relatively easily implemented cross-flow pattern, that is, where the fuel and oxidant gases are introduced into the stack in a cross-flow configuration. Such high temperatures and high temperature gradients increase mechanical stress and accelerate deterioration of solid oxide fuel cell stacks. Smoothing the in-plane stack hardware temperature gradient would have the potential to increase stack life.
U.S. Patent Publication No. 0207163-A1 teaches the use of air preheater panels as a means for addressing thermal management issues encountered in fuel cell stack systems. However, successful thermal management in the disclosed system generally requires the use of at least one pre-burner for boosting the temperature of the air leaving the air preheater panel and entering the fuel cell stack.
Control of a system having modules comprising a combination of fuel cell stack and gas flow panels requires appropriate thermal interaction among the system components. For example, an oxidant outlet heat exchanger may need to operate independently of the gas flow panels so that the panel temperature can stay low enough to maintain effective radiant heat transfer from the fuel cell stack. This may be achieved by locating a control burner for preheating inlet gases remotely from the panels. Remote location of a supplementary burner would also allow for controlling the thermal gradients developed in the stack by controlling the transfer of stack-generated heat alone. An air bypass can be used to assist system control.